The Formation of a 70 Msun Black Hole at High Metallicity

TL;DR

This paper demonstrates that under reduced stellar wind mass loss rates, high metallicity stars can form black holes up to 70 solar masses, challenging previous limits and explaining recent observations.

Contribution

It introduces models showing that decreased stellar wind mass loss allows formation of very massive black holes at high metallicity, supported by detailed stellar evolution simulations.

Findings

A 70Msun BH can form at high metallicity with reduced stellar winds.

Stars capable of forming such BHs can reach large radii, exceeding 600 solar radii.

The formation scenario explains recent BH observations but challenges binary formation theories.

Abstract

A 70Msun BH was discovered in Milky Way disk in a long period and almost circular detached binary system (LB-1) with a high metallicity 8Msun B star companion. Current consensus on the formation of BHs from high metallicity stars limits the black hole mass to be below 20Msun. Using simple evolutionary model, we show that the formation of a 70Msun BH in high metallicity environment is possible if stellar wind mass loss rates are reduced by factor of 5. As observations indicate, a fraction of massive stars have surface magnetic fields which may quench the wind mass-loss, independently of stellar mass and metallicity. We also computed detailed stellar evolution models and we confirm such a scenario. A non-rotating 85Msun model at Z=0.014 with decreased winds ends up as a 71Msun star prior core-collapse with a 32Msun helium core and a 28Msun CO core. Such star avoids pair-instability pulsation supernova mass loss and may form a 70Msun BH in the direct collapse. Stars that can form such BHs expand to significant size with radius of R>600Rsun, exceeding the size of LB-1 orbit. Therefore, we can explain the formation of BHs upto 70Msun at high metallicity and this result is independent from LB-1. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios.